For various applications, including surveying, engaging in combat, and tracking objects, it is often necessary to determine the distance of remotely located objects and to communicate with a third party that may be proximate to the remotely located object. Conventionally, such functionality has been achieved using two separate systems that have been either physically combined into a single operating unit or separately provided into two physically distinct operating units.
For example, one approach to determining a distance to a remote object is to measure the time of flight of a pulse of light from the measuring system to the object and back again, and, subsequently, to calculate the distance to the object based upon the speed of light and differences in the transmitted and received light. Systems employing this method typically employ a laser to generate the light pulse and are known generically as “laser range finders” (LRFs) or “light detection and ranging” (LiDAR) systems.
The accuracy and resolution of a LRF system, along with the maximum range that can be dependably measured by such systems, are dependent on a plurality of factors. For example, the laser pulses received after reflection from a target comprise a replica of the transmitted laser pulse signals along with undesirable signals or noise making it difficult to detect the signals of interest. Further, the laser pulses transmitted and received after reflection from the target undergo significant attenuation due to factors such as the nature of the surface of reflection of the target, atmospheric conditions and the distance of the target from the ranging system. Typically, LRF systems optimize operational efficiency by using high powered lasers, such as YAG solid state lasers, that transmit using intermittent pulses of high power, such pulses often being separated by a minimum recharge period of several milliseconds.
Data communications may be enabled by numerous devices and systems, from satellite transceivers, to mobile phones, to conventional PSTN connections, to packet-based network communications. Applications that require a high degree of data security and must operate in situations and locations that may not have a reliable, or even an existent, telecommunications infrastructure have conventionally relied on some form of point to point optical communication. Such systems use a optical transmitter, such as a laser, in combination with a retroreflector to receive a laser transmission and modulate a return signal.
These conventional optical communication systems have substantial disadvantages, however. Because an optical communication system needs to achieve a sufficiently high data rate in order to be able to modulate, and therefore communicate, a meaningful amount of data, these systems rely on lasers having very different operational characteristics, as compared to lasers conventionally used in LRF systems. Specifically, conventional optical communication systems use lower power, higher pulse rate lasers that require a fraction of the recharge time required by higher power lasers. Consequently, such systems can not be integrated with conventional laser range finding systems.
Additionally, high data communication rate requires responsive retroreflectors in order to enable a sufficiently high modulation rate. Without a sufficiently responsive retroreflector, high modulation rates can not be achieved and, consequently, high data communication rates can not be effectuated, even if the appropriate laser system is utilized. U.S. Pat. No. 4,887,310 discusses identification devices having means for modulating a reflected laser beam at the target. However, the disclosed systems are very expensive and not sufficiently sensitive. U.S. Pat. No. 4,134,008 discloses a method for transmitting an a response code to an interrogation code received at the location of the retroreflector. Kerr or Pockels cells, or PLZT ceramics (lead-lanthanum-zirconium-titanium), are used in the abovementioned system for modulating the retroreflector signal. These modulators require high operating voltages and either very costly or allow for a relatively low modulation frequency.
In light of the abovementioned disadvantages, there is a need for an integrated optical communication system facilitating remote data communications and for a laser range finding system for determining the distance of a remote object. There is also a need for enabling the use of a single laser system for both laser range finding and optical communications. Furthermore, there is a need for retroreflector systems, and methods of use, that enable the sufficiently fast modulation of a data signal.
There is also a need to have an integrated laser range finding and data communication system adapted for use in military operations, particularly in secure covert operations. Military forces have an interest in the remote and secure identification of a person, during combat training exercises and in armed conflicts, and in the tracking and identification of objects. Identification as friend or foe (IFF) systems are well-known in the art for military aircraft and other weapons systems. Such systems are useful for preventing action against friendly forces. The military platform commanders on a modern battlefield must accurately identify potential targets as friend-or-foe (IFF) when detected within range of available weapon systems. Such target IFF presents a difficult decision for a military platform commander, who must decide whether to engage a detected target while avoiding accidental fratricide. This problem is even more difficult for the dismounted soldier who may be moving covertly through an unknown combat zone at night with limited visibility. Simple visual assessments of other dismounted soldiers is not a reliable IFF method for military platforms or dismounted infantry.
The art is replete with proposals for IFF systems for military platforms in modem land battlefields. But commanders often still rely on low-resolution visual and infrared images to identify detected targets. Commanders often must operate under radio silence to avoid detection by an enemy. With infrared (IR) imagers alone, the identification of individual dismounted soldiers is not feasible, although the IR signatures of land vehicles may have some use. IFF systems that require one or more radio signals are limited in channel-capacity and must bear the overhead of selecting and/or awaiting an available battlefield channel before completing the IFF task. Active-response systems require the emission of a signal by the unknown respondent in response to a verified challenge, which may compromise the security of both interrogator and respondent. Active transponders are subject to capture and may be used for spoofing by the enemy in a battlefield or a combat training environment. Passive response systems rely on the return of an echo (reflection) of a challenge signal to the interrogator, but simple reflection schemes are easily compromised and more elaborate passive reflection schemes are still subject to intercept, compromise or capture for use by the enemy in spoofing the interrogator.
As described in U.S. Pat. No. 4,851,849 by Otto Albersdoerfer, a typical active IFF technique for vehicles is to equip a military vehicle with a transponder that emits a coded return signal when an interrogating radar pulse is detected by its receiver. As described in U.S. Pat. No. 5,686,722 by Dobois et al., a more sophisticated active IFF technique for vehicles uses a selective wavelength optical coding system with tunable optical beacons mounted on each vehicle. By spreading the optical broadcast energy into frequency in a precise manner, the beacon identifies the host vehicle to friendly receivers while remaining covert to the enemy.
As described in U.S. Pat. No. 4,694,297 by Alan Sewards, a typical passive IFF technique for vehicles is to equip a military vehicle with a passive antenna that reflects an interrogatory radar beam while adding a distinctive modulation by varying the antenna termination impedance responsive to evaluation of the interrogatory beam. A more sophisticated passive electro-optical IFF system for vehicles is described in U.S. Pat. No. 5,274,379 by R. Carbonneau et al. wherein each friendly vehicle is provided with a narrow-beam laser transmitter and a panoramic detector. If a vehicle detects a coded interrogator laser beam and identifies the code as friendly, it opens a blocked rotating retro-reflector to clear a reflection path back to the source, where it can be identified by another narrow field-of-view detector. A further modulation is also added to the reflected beam to identify the reflecting vehicle as friendly. If an improperly coded beam is detected, the transmission path is not cleared, thereby preventing reflection of that beam and warning is sent to the vehicle commander of an unfriendly laser transmission. Others have proposed similar passive optical IFF systems for vehicles, including Wooton et al. in U.S. Pat. No. 5,459,470 and Sun et al. in U.S. Pat. No. 5,819,164.
The art is less populated with IFF proposals for the lone dismounted soldier (the infantryman on foot). Whether in actual combat or in a training exercise, the dismounted soldier operates with severe weight limits and little onboard electrical power. The friendly foot soldier has no distinctive acoustic, thermal or radar cross-section that may be used to assist in distinguishing friendlies from enemies. But some practitioners have proposing IFF solutions for the dismounted soldier, both active and passive. For example, in U.S. Pat. No. 6,097,330, Kiser proposes an active IFF system for identifying concentrations of ground troops (or individuals) from an aircraft by interrogating a (heavy) human-mounted radio transmitter carried by one of the group with a narrow-cast optical signal. As another example, in U.S. Pat. No. 5,299,277, Rose proposes a compact active IFF system to be carried by each individual dismounted soldier for use in combat exercises or on the battlefield. The system includes a clip-on beacon and a hand-held (flashlight-style) or weapon-mounted detector. The beacon radiates a spread-spectrum low-probability-of-intercept (LPI) signal at optical frequencies that are selected to be invisible to the usual detectors present in the battlefield. Rose doesn't consider the problem of spoofing with captured devices. As yet another example, in U.S. Pat. No. 5,648,862, Owen proposes an active IFF system implemented by adding provisions for coded two-way transmissions to the night-vision systems often worn by dismounted soldiers. As a final example, in U.S. Pat. No. 5,966,226, Gerber proposes an active combat IFF system for each dismounted soldier that includes a weapon-mounted laser projector for interrogating suspected targets and a harness including means for receiving the interrogatory signal and means for responding with an encoded radio, acoustic or optical signal.
These proposals do not resolve, however, the spoofing problem (through capture of a beacon or harness, for example); and are not particularly covert because the responding target generally broadcasts an active signal either continuously or in response to interrogation. Any IFF proposal employing broadcast signals also faces a battlefield channel capacity (or channel availability delay) problem as well. Furthermore, these proposals do not address the need to be able to actually communicate with a target to verbally identify the individual as being a friend or foe.
There is still a need in the art for a secure cover system (SCS) for that provides true passive covertness and that cannot be spoofed under any battlefield conditions. The desired SCS system requires little power and is adapted to prevent any use of captured equipment or intercepted signal codes. Furthermore, the desired SCS system is capable of enabling both range finding and optical communication functionality in the lightest weight configuration possible. Finally, the system should be inexpensive enough to permit equipping every soldier with the necessary interrogation and response equipment for combat exercises or actual battlefield conditions.
These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.